The human calcitonin receptor (CTR) is a G protein-coupled receptor that is an important drug target because of its role in mediating the distinct biological actions of two related endocrine peptide hormones, calcitonin (CT) and amylin (AMY). CT regulates calcium homeostasis and bone turnover by signaling through the CTR. AMY regulates blood glucose levels by signaling through a heterodimeric complex ofthe CTR and any one of three related receptor activity modifying proteins (RAMPs) that act as co-receptors to alter CTR specificity for AMY. Agonism of the CT and AMY receptors by synthetic analogs of the hormones is used to treat osteoporosis and types I and II diabetes, respectively. Despite the clinical value of CT and AMY receptor agonism, the molecular mechanisms of CTR hormone binding and RAMP-mediated alteration of CTR hormone specificity are poorly understood because of a lack of structural information for the hormone-receptor complexes. CT and AMY binding affinity and specificity are in large part determined by the extracellular domains (ECDs) ofthe CTR and RAMP integral membrane proteins. The goals of this proposal are to characterize the hormone-receptor ECD interactions and to determine crystal structures of soluble CT- and AMY-receptor ECD complexes. The results will define the molecular bases for CT and AMY binding to their receptor ECDs, delineate how RAMPs alter hormone specificity ofthe CTR, and provide structural templates to guide the design of optimized therapeutics targeting the receptors. We propose the following three aims: (1) Determine the molecular mechanism of CT recognition by the human CTR ECD. The crystal structures generated in this aim will define how human and salmon CT hormones bind to the CTR and aid the development of therapeutics for treating osteoporosis. (2) Determine the structural and functional bases for interaction ofthe CTR and RAMP ECDs. The resulting CTR ECD-RAMP ECD heterodimer structure will reveal the molecular architecture of an AMY receptor. (3) Determine the molecular mechanism of AMY binding to a CTR ECD-RAMP ECD heterodimer. Achieving this aim will define how a RAMP alters the hormone specificity of the CTR and aid the design of therapeutics for diabetes.